The design and construction of vibration isolators, absorbers, and damping treatments has led to important advancements in vehicle suspension technology and other applied fields. Traditionally, vibration control is achieved using passive elements such as springs, dampers and masses in the form of metallic, pneumatic, hydraulic or rubber devices. These elements are considered "passive" in the sense that no power source is required for their effective operation, i.e., the vibration control elements only store or dissipate the energy associated with the vibratory motion. As a practical matter, numerous vibration problems can be solved using inexpensive, reliable passive devices; however, distinct performance limitations are inherent when only passive devices are used, which may be unacceptable for more sophisticated applications. Attempts at improving these vibration control devices have included the provision of adjustable parameters which could be varied to suit changing excitation or response characteristics of the particular system. For example, automobiles have utilized manually adjustable shock absobers. More recently, modern vehicle suspensions and isolation systems have incorporated more elaborate devices such as automatic leveling systems to adjust static deflection as the suspension load varies. However, the most notable shortcoming of any such passive device is that in addition to failing to provide sufficient damping force during certain instances of dynamic response, damping forces may at other times tend to amplify, rather than attenuate the transmission of energy to produce an undesirable effect.
Fully active dampers or systems have been created which utilize high pressure pumps, high performance servo-mechanisms or other external energy source to vary vibration control system parameters. With the advent of optimization in control theory and development of an understanding of the dynamics of such systems using these devices, fully active systems have been constructed for a wide variety of applications which include vehicle suspensions, helicopter rotor isolation, flexible aerospace vehicle bending mode control and the isolation of pilots from aircraft motion. Although fully active vibration control systems can achieve excellent performance, their expense, complexity, bulk and unreliability may have, in many instances, made them commercially unsuitable for many applications.
Semi-active systems have been created in which much of active system performance gains are relized without the attendant complexity, cost and external power or drive requirements, which are further clearly superior to previous adjustable or static passive systems. Semi-active dampers utilize no hydraulic pump or large external drive means, yet in contrast to purely passive systems, are rapidly switchable between damping states to vary the damping coefficient. According to the particular control policy utilized to drive a semi-active damper, attenuation of the transmission of energy between movable members can be achieved which approaches that realized in fully active systems. Semi-active dampers, and in some instances control policies for them, are disclosed in U.S. Pat. Nos. 3,807,678; 3,995,883; 4,468,050; 4,468,739; 4,491,207; 4,696,489 and 4,742,998; and also in U.S. patent application Ser. Nos. 913,067 filed Sept. 29, 1986, and 945,380 filed Dec. 22, 1986, both owned by the assignee of the present application. The disclosures of the aforesaid patents and applications are incorporated herein by reference.
In certain applications, operation of real time semi-active damping systems which produce relatively abrupt changes in damping coefficient under normal working conditions can result in the generation of undesirable impulses or shock forces that lead to unwanted system stress and noise. This shock or noise generation problem may, in certain instances, be eliminated by reducing the differential between the damping coefficient high and low states, but this may degrade performance of the damper to an unacceptable extent. Other possible solutions include electronically conditioning the control signals imparted to the flow-controlling valve means of the damper to cause it to operate more slowly, or to delay its effecting switching of the damper status until the relative velocity between the members interconnected by the damper, and thus "across" the damper itself, is at a selected low value. However, for the operation of semi-active damping systems in certain environments, more reliable and cost effective performance is better achieved by decreasing rather than increasing reliance upon electronic system monitoring or control devices. Commercially competitive systems might seek to minimize the sensing of system motion parameters and the frequency of valve commands.
Another characteristic of semi-active systems, especially in vehicle applications, is the repeated, rapid transition of the actuated valving elements required to appropriately modulate the damping coefficient. In such cases, damper switching takes place to a large degree based on changes in the sign of the relative velocity between vehicle body and frame components. For control policies based at least in part upon this relative velocity parameter, commanded valve switches due to a relative velocity sign change are necessitated with a much higher degree of frequency than valve switches dependent upon other system motion conditions. In addition to system fatigue and wear which can be associated with the valve elements laboring under multiple switching commands, motion sensing and signal conditioning requirements for these control policies or modifications to them can augment the cost and unreliability of such systems.
It is accordingly an object of the present invention to provide a semi-active damper means which eliminates or substantially minimizes the above mentioned and other problems and limitations typically associated with semi-active devices of conventional construction and operation.